Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines, and offer a stepless control of the machine's speed. A typical hydrostatic transmission system includes a variable displacement main hydraulic pump connected in a closed hydraulic circuit with a fixed displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral. For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Many hydrostatic transmissions also include a charge pump for providing makeup flow to the closed circuit to replace fluid lost due to internal leakage. Charge pumps are typically positive displacement pumps (e.g., a gear pump) driven by the same input as the main hydraulic pump and provide fluid to the closed circuit via one or more check valves. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is typically connected through suitable gearing to the vehicle's wheels or tracks.
In many applications, fluid from the hydrostatic transmission is used to operate one or more auxiliary hydraulic circuits, for example, to raise and/or lower an implement such as a mower deck. Such auxiliary circuits, though used relatively seldom, often require relatively high pressure to operate properly. Thus, a readily available high pressure fluid source is needed to power the auxiliary circuit.
One solution has been to provide a hydrostatic transmission with an auxiliary pump capable of providing sufficient pressurized fluid to operate the auxiliary circuit. In some applications, the auxiliary pump is provided in place of a charge pump and is used to supply fluid not only to the auxiliary circuit, but also to provide makeup flow to the closed loop of the hydrostatic transmission. Due to the high pressure required by the auxiliary circuit and the infrequent use of the same, such an arrangement generates significant heat as the auxiliary pump continuously pressurizes fluid. Accordingly, such systems often require an additional fluid cooler for dissipating the generated heat.